1. Field of the Invention
This invention relates to a process for producing granular abrasives having improved grain toughness. More especially, this invention relates to a simple and improved process for obtaining granular abrasives with an improved grain toughness such that they can be used for the grinding of castings, steel ingots and the like.
2. Discussion of the Prior Art
Abrasive grits which are made into grinding bodies, chiefly for the rough grinding of castings, steel ingots or the like, consist mainly of corundum. For some years fused or sintered zirconium corundums have been used preferentially in this field of application, since they are characterized by greater grit toughness and therefore result in greater grinding efficiency.
Grit toughness is on the one hand a characteristic of the material itself, but on the other hand it is also dependent upon the form of the individual grain. Thus, for example, a grain having rounded corners and edges is tougher than a splintery grain. However, pores and voids within a grain reduce its toughness in comparison with the toughness of a compact grain. The crystal structure of the individual grain is also a determining factor in grit toughness. A grain having a coarse crystalline structure is substantially less tough than one having a fine crystalline internal structure.
Grit toughness is known to determine to a considerable extent the resistivity of a grinding wheel. The term "resistivity" in connection with a grinding wheel, as used herein, is to be understood as the weight ratio between the material abraded from the workpiece and the loss of weight from the grinding wheel.
A variety of processes have been proposed for the production of finely crystalline abrasive grits of high grain toughness for use in making grinding bodies of high resistivity.
According to German Pat. No. 1,571,435, the process consists of melting together zirconium oxide and aluminum oxide and/or bauxite, avoiding contact with nitrogen insofar as possible during the casting and solidification of the melt. By this method, abrasive grits of great grain toughness are obtained. A zirconium corundum prepared in this manner not only has fewer pores and voids within the grain, but its structure is decidedly more finely crystalline than that of a zirconium corundum prepared by conventional methods.
In other known processes for the production of finely crystalline abrasive grits of high grain toughness, for manufacture into grinding bodies of high resistivity, a molten mass of the desired composition, consisting of pure aluminum oxide, a mixture of aluminum oxide and chromium oxide, aluminum oxide and titanium oxide, aluminum oxide and zirconium oxide, and possibly other substances or combinations of these substances, is rapidly chilled by pouring it from the crucible. To accelerate cooling a number of methods have been proposed. For example, according to U.S. Pat. No. 1,192,709, rapid cooling can be achieved by pouring the melt into small molds. Other methods are based on controlling the rate of cooling by pouring the melt into molds filled, for example, with iron balls or with cold pieces of the same material.
The processes known hitherto for the production of finely crystalline abrasive grits are encumbered, however, by the disadvantage that abrasive grits produced by such methods contain to some extent large numbers of very fine pores and voids or other flaws, such as tensions in the crystal structure, for example, in spite of a finely crystalline internal structure. Some amounts of coarsely crystalline material can also be found in these grits. Such grits result in a reduction of the grain toughness of the abrasive and hence also of the resistivity of a grinding body.
It has, therefore, become desirable to provide a process for the production of finely crystalline abrasive grits which are free of granular materials containing very fine pores, voids or other flaws and which are substantially free of grains having tensions in the crystal structure. It has become particularly desirable to provide such a process which is not dependent upon avoiding contact of the elements of the melt with nitrogen, nor is it dependent upon the method by which the refractory material is prepared or cooled down from the melt.